The present invention relates in general to coating methods and apparatuses, and in particular to a new and useful method and apparatus for regulating the ratio between a solid and gaseous component of material to be coated onto a workpiece.
In the production of layers or coatings with a plasma-enhanced reactive deposition process, wherein reactive cathode sputtering at least participates, it is known to regulate the degree of reaction .gamma., to be a constant value or, in the sense of a master control, to obtain a profile of the degree of reaction along the layer thickness.
In control processes of this type, the regulating parameter is consequently the particular stoichiometric relation on the growing layer according to the stated ratio .gamma..
It is therein known to determine the so-called "measured regulating parameter" which deviates from the actual regulating parameter and from which, based on the known regularities, conclusions are drawn about the first, for example by means of plasma emission monitors, by means of quartz microbalances, mass spectrometers, etc.
As the so-called manipulated parameter, it is customary to set the sputtering rate of the cathode evaporation or the reactive gas flow into the vacuum chambers. Depending on the intended coating process, the cathode sputtering process can therein be operated with DC, DC and superimposed AC or with pure AC, such as for example Hf cathode sputtering.
It has been found that the above stated approach is often problematic in terms of regulation technology, inter alia due to instabilities of the regulation. This can be traced back, inter alia, to the fact that in the addressed reactive cathode sputtering the technical regulation path also forms with the sputtered cathode, the reaction process a time-variant regulation system. It does this because the reaction process influences also the surface of the sputtering cathode and therewith the behavior of the latter as a regulation path element. In this connection the phenomena of target contamination with insulating layers or islands is well known and the topic of an extensive literature.